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July Sizzles, Records Fall: Warmest Month on Record

July 2012 was officially not only the warmest July on record, but also the warmest month ever recorded for the lower 48 states, according to a report released Wednesday by the National Oceanic and Atmospheric Administration's National Climatic Data Center. The average temperature for the month came in at 77.6°F overall, which is 3.3°F higher than the 20th-century average, and 0.2°F warmer than the previous hottest month on record, set in July 1936 back in the Dust Bowl era.

The warmest 12-month periods for the lower 48 states. Click on the image for a larger version. Credit: NOAA/NCDC.

It wasn’t just that July was a single-record month: the 12 months ending with July was the warmest such period since modern recordkeeping began in 1895, and the January-July 2012 period was also the warmest on record. The top 13 warmest 12-month periods since 1895 have all occurred since 1999.

The National Climatic Data Center also looked at precipitation: the average for July was 2.57 inches, which was 0.19 inches below average. That may not sound like much of a shortfall, but the nation’s midsection experienced near-record dryness.

Overall, the so-called drought footprint for the states, excluding Alaska and Hawaii, covered nearly 63 percent of the total land area, according to the U.S. Drought Monitor. The drought is the most widespread and intense drought since at least 1956, and is expected to cost billions in damage to agricultural interests, as what was expected to be a bumper corn crop withered under unrelenting heat and dry conditions.

Extreme weather continued to plague the nation as well. The U.S. Climate Extremes Index, which keeps track of the highest and lowest extremes in temperatures, precipitation, and other events, stood at a record 46 percent for the period January-July, 2012, which is twice the average. That means that nearly half the country was affected by extreme weather conditions during the period. The record (42 percent) was last set in 1934 — again, during the Dust Bowl.

Year-to-date Climate Extremes Index, showing that a record area of the U.S. has been affected by weather and climate extremes so far in 2012 compared to all other years since such records began in 1910. Click on the image for a larger version. Credit: NOAA/NCDC.

Much of the explanation for the currently high index is due to very warm daytime temperatures and warm overnight temperatures across a record-large area of the nation. The overnight warmth is what distinguishes July, 2012 from July, 1936. "In 1936," said NOAA scientist Jake Crouch in an interview, "the record was driven primarily by high daytime temperatures." In both cases, the daytime highs were driven in part by drought. When the soil is wet, Crouch said, "solar energy goes into evaporating moisture." When it's dry, the same energy goes into raising the thermometer.

Warm nights, however, don't have much to do with soil moisture, so they're a more robust signal that the planet is warming overall in response to increasing concentrations of greenhouse gases. They're also more dangerous than daytime heat. "Cooler temperatures at night let our bodies recover," Crouch said.

As Climate Central reported yesterday, record daily high temperatures through August 5 of this year have already eclipsed the number of record daily highs set during all of 2011, a remarkable feat. For the year so far, record daily highs have been outpacing record daily lows by a ratio of nearly 10-to-1, which reflects the trend identified in a 2009 study on how temperature records are changing as the climate warms.

Some of the other highlights in the report:

— The largest departures from average temperatures occurred across most of the Plains, the Midwest and along the Eastern Seaboard.

— July temperatures in 32 states were among the top 10 on record. Seven had their second-warmest temperatures, and Virginia had its warmest July.

— Nebraska, Iowa, Illinois, and Missouri had precipitation totals among the 10 lowest on record. These states include some of the nation’s prime cropland; a broader swath of agricultural states known as the Primary Corn and Soybean Agricultural Belt, had its eighth driest July, third driest June-July, and sixth driest April-July (which covers the entire growing season so far) on record.

— July’s heat and dryness created ideal conditions for wildfires, and more than 2 million acres burned during July, nearly 30 percent above average.